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    Analysis of adhesion mechanism and influencing factors of waterborne coatings

    • Last Update: 2022-11-15
    • Source: Internet
    • Author: User
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    Good adhesion of the coating to the surface to be coated is the premise
    for the coating film to exert its protective role.
    The adhesion of the coating is one of the most important indicators of
    the coating during application.

    Mechanism of adhesion

    The importance of adhesion has long been known, but until now, the mechanism of adhesion in coatings has not been fully unders coatingol.
    Most explanations of adhesion are based on experience and hypotheses, but we can also use it as a means to make dialectical arguments

    Mechanical adhesion

    The mechanism of this adhesion is mainly for coating on substrates with uneven interfaces containing holes, holes or cracks
    When the paint liquid is applied to the surface of the substrate, the paint can wet and penetrate into these gaps, so that after the paint film is cured, the paint film that enters the gap can act like a nail, thereby grasping the substrate

    The mechanism of mechanical adhesion has certain manifestations
    in such as wood, sandblasting, mechanical grinding, etc.

    Covalent bond theory

    The theory of covalent bonding refers to the formation of certain covalent bonds after the coating film comes into contact with the substrate, and this type of link force is
    the strongest and most durable.
    The premise of formation requires that there are chemical groups that fit each other in the coating film and substrate, and these groups can form chemical bonds
    when they are close together.

    For example, the adhesion of some organosilanes to glass, metals and organic ores is enhanced; Another example is that some substrates have hydroxyl groups, carboxyl groups and the curing agent in the coating.

    Van der Waals force theory

    Van der Waals forces are the most common force
    that provides conventional adhesion.
    When the coating film is in close contact with the substrate, the molecules are close to each other and create attraction

    This attraction is a short-range interaction, generally with a strong effect
    at 0.
    In addition, the larger the relative molecular mass of the molecule, the greater the deformity, and the easier it is to be polarized, the stronger the force

    Electrostatic theory

    There is a certain residual charge dispersion on both the surface of the coating and the substrate, and when the paint and the substrate are in close contact, these charges attract each other to promote the adhesion
    of the paint film.

    Hydrogen bonding theory

    When hydrogen atoms form covalent bonds with atoms with large electronegativity (such as O, N, F, etc.
    ), the electrons are biased towards atoms with high electronegativity, and hydrogen atoms are positively charged

    Therefore, when the coating contains groups similar to -OH and -COOH, and when the substrate contains negative electric groups, a hydrogen bonding effect may be formed, thereby improving the adhesion
    of the coating film.

    This type of hydrophilic group may affect the water resistance of the coating film

    Diffusion theory

    When the coating film is applied to the substrate, the two affinity macromolecules may form a staggering through diffusion, which improves adhesion
    This force can only occur between the coating and the substrate that are compatible with each other, so it mostly acts on the adhesion between layers, especially between the two coatings of the unified system

    The above is a common explanation
    of the mechanism of adhesion formation.
    In fact, in the coating of coatings, the mechanism of adhesion is diverse, often containing a variety of forces

    Influencing factors of adhesion

    From the above analysis of various mechanisms, it can be seen that regardless of the form of force to promote adhesion, the close adhesion of the coating to the substrate is the primary condition
    By tightly bonding and reusing a special attachment structure of resin or additives, a strong and tough paint film
    adhering to the substrate can be obtained.

    It can be seen that different aspects such as the performance of the resin itself, the formula structure, and the coating will have a certain degree of impact
    on the adhesion.

    Characteristics of water-based resins

    When it comes to performance influencing factors, the first thing to talk about is the performance effect of the waterborne resin itself, and adhesion is no exception
    And the performance characteristics of waterborne resin itself are the most important influencing factors
    of adhesion.

    The wetting fluidity, particle size, special groups, surface energy and cohesion after forming a dry film of waterborne resin will greatly affect the adhesion of
    the coating film.

    Take SV-6145 with excellent adhesion as an example:

    (1) The resin has good wetting fluidity, and through good wetting, flow and spreading, the paint film can be in closer contact
    with the substrate.

    (2) The resin particle size is small, which prevents the steric hindrance effect formed by excessive latex particles, affecting the effective contact between the paint film and the substrate, simply put, the film formation is more dense

    (3) It also contains a special anchor group and has a low cohesion
    Through the linkage effect of special groups, excellent adhesion is formed, which is enough to offset the effect
    of its low cohesion on the coating film.
    In general, softer resins have more advantages in adhesion, because they are more able to adapt to different degrees of deformation of the substrate as the environment changes

    SV-6145 still ensures good adhesion when it has excellent hardness

    Substrate treatment

    Substrate treatment, such as grinding, sandblasting, etc.
    , can effectively increase the roughness
    of the substrate.
    The increase in roughness can greatly improve the contact area between the coating film and the substrate, and provide many effective anchoring points, which is conducive to improving the adhesion between the coating film and the substrate

    The surface tension of the paint solution

    When the paint is applied to the surface of the substrate, the old interface disappears and a new interface
    is formed.
    During the coating process, the degree of wetting of the coating to the substrate determines whether the coating film can form effective contact
    with the substrate.

    Due to the high surface tension of the water, wetting spreads on low-surface energy substrates are problematic, which also leads to some problems

    Therefore, how coatings or waterborne resins can achieve low surface tension and excellent wetting spreadability is particularly important

    When designing the formulation, the addition of substrate wetting agent should also be ensured to ensure that the coating can effectively and excellently wet the substrate

    Coating thickness

    Many experiments have shown that adhesion gradually decreases
    as the coating film thickness increases.
    Mainly because as the thickness of the coating film increases, its cohesion also increases, but the adhesion remains the same; When the cohesion increases greater than the adhesion, then the coating will have poor

    From the perspective of the protective performance and fullness of the coating, the coating film should have a certain thickness, so a balance
    should be made between these two aspects.

    In addition, with the same total film thickness, the adhesion of multiple coatings is better
    than that of one coating.

    Drying process

    During the drying process, ambient temperature and humidity have a certain influence
    on adhesion.

    The above is to test the influence of
    temperature and humidity on the adhesion of a polyurethane water-based paint on glass.

    In this experiment, the maintenance conditions of higher temperature and lower humidity have a certain promotion effect
    on adhesion.

    Paint viscosity

    The viscosity of the paint solution will also affect the adhesion of the coating film to a certain extent, one is from the influence on the wetting spread, and the other is that the inappropriate viscosity will form stress to weaken the adhesion

    Adhesion promoter

    Certain adhesion promoters significantly improve the adhesion of paints, which are formed by a special chemical structure

    According to different substrate types, different additives have different effects, common types such as epoxysilane

    Face filler

    A small amount of pigment filler can generally increase the adhesion of the coating, the principle is to reduce the cohesion energy
    of the system.
    Different pigments have different effects, for example, the improvement effect of scale-like pigments is more obvious

    When the amount of pigment filler reaches a certain level, the adhesion will decrease
    with the increase of the amount added.


    Sometimes, solvents also have a significant effect on film adhesion, which is produced
    by surface tension, drying and plasticizing effect on the paint film.
    Most solvents have a relatively low surface tension, which is helpful for the wetting spread of water-based paint on the substrate and is conducive to improving the adhesion
    of the coating film.

    There are also solvents that evaporate slowly and can take weeks or even months to evaporate cleanly
    When the solvent remains inside the paint film, the adhesion
    of the coating film itself is weakened.

    In addition, when the solvent gradually volatilizes after the paint film is cured, the paint film may shrink to a certain extent in this process, which will produce a certain degree of internal stress, resulting in poor adhesion

    It can be seen from the above that the formation of coating adhesion and the influencing factors are multifaceted
    When encountering adhesion problems, you should think
    comprehensively about the problem.

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